Perfluoropolymer resins are known for their low surface energy and resultant low coefficient of friction and non-stick properties, as well as thermal and chemical resistance. Polytetrafluoroethylene (PTFE) has the lowest coefficient of friction of the perfluoropolymers, and is thus finds wide utility as a non-stick surface. However, PTFE presents processing difficulties and costs due to its very high molecular weight and as a result not being melt flowable and fabricable, and by having a relatively much higher melting point (˜326° C.) than other perfluoropolymers. Fluorinated ethylene propylene (FEP, tetrafluoroethylene/hexafluoropropylene copolymer) is beneficially melt flowable and melt fabricable and has a lower melting point (˜260° C.) which affords more manufacturing flexibility. However, FEP has a relatively higher coefficient of friction than PTFE, and so articles having a FEP surface will not afford quite as low a coefficient of friction and non-stick properties as articles surface coated with PTFE.
Due to their non-stick nature, it is also difficult to adhere perfluoropolymers to commercial substrates such that the perfluoropolymer coating does not separate or delaminate from the substrate upon use. There are a variety of “primer” or “binder” polymers having good adhesion to substrates as well as good adhesion to perfluoropolymers, and often a primer layer of such polymers is deposited on a substrate followed by a perfluoropolymer layer deposited on and adhered to the primer layer. The combination layered coating is known in this field as a “two coat” coating, being well adhered to the substrate by the primer polymer and also well adhered one polymer layer to the other. When the presence of a primer layer is not preferred, a “one coat” coating may be used. The one coat coatings involve a single coating of a composition that is an intimate physical mixture of perfluoropolymer and such binder polymer. The specific materials and their relative amounts are determined based on consideration of the substrate and the ultimate coating utility and properties desired.
Further complicating use of perfluoropolymer coatings is that certain polymer and metal substrates have relatively low limits of acceptable temperature exposure. In applications where such substrates would benefit from a perfluoropolymer coating, the low limit of acceptable temperature exposure of the substrate complicates and thus increases the costs of, or makes impossible, coating of such substrates or their articles with a perfluoropolymer coating by a thermal process. In the instance where the perfluoropolymer has a melting point above or near the limit of acceptable temperature exposure of a substrate, coating of the substrate by such molten perfluoropolymer, or coating of the substrate with perfluoropolymer particles (e.g., by solution coating or powder coating techniques) following by baking of the perfluoropolymer coating at or above the melting temperature of the perfluoropolymer can irreversibly damage the substrata. Here, “baking” and “baking temperature” refers to treatment of the coating at a temperature that results in the perfluoropolymer particles coalescing into a uniform and continuous coating upon melting, melt flow and mixing of the molten perfluoropolymer.
As an example, nickel titanium alloy (nitinol) is a simple binary mixture of nickel and titanium containing about 50 atomic percent each (about 55 percent by weight of nickel). Nitanol is stable against permanent temperature-induced metallurgical changes provided the exposure temperature is less than the annealing or aging temperatures. For many nitinol alloys, the aging temperature range is from 200° C. to 500° C. As the aging temperature of nitinol can fall below the melting point of the perfluoropolymers, conventional melt processing methods (e.g., melt extrusion, powder coating followed by baking near or above the perfluoropolymer melting point) are not available and as a result it is a challenge to coat nitinol surfaces with continuous low coefficient of friction coatings of perfluoropolymer. A like challenge exists for similarly coating polymers having melting point near or below the melting point of the perfluoropolymers.
A specific problem faced in producing articles for use in the medical field is the coating of a perfluoropolymer onto nitinol wire or the like to improve the surface friction characteristics of the wire (e.g., a medical wire such as a cardiac catheter guide wire), or using a pigment-containing perfluoropolymer as the surface layer of a medical wire so that medical care professionals are able to identify and differentiate one from another medical wires using only the color of, or patterned external design of, the wires. Perfluoropolymer coatings will typically exhibit superior low surface friction properties only after baking at a temperature at or above the perfluoropolymer melting point. For this reason, in the process for their manufacture, such medical guide wires coated with perfluoropolymer are typically subjected to a baking treatment for a period of time where the baking temperature is at or above the melting point of the perfluoropolymer. However, such a method has a problem in that the physical properties (e.g., elastic modulus) of the nitinol wire is negatively impacted when the perfluoropolymer-coated medical guide wire is baked. Moreover, when such a perfluoropolymer-coated medical guide wire is baked, if the perfluoropolymer additionally contains a colored pigment, it is undesirable for the color of the pigment to fade or undesirably change color and thereby not produce the desired medical guide wire having a colored outer jacket.
There remains a commercial need for low coefficient of friction fluoropolymer one coat coatings having a low bake temperature allowing for uniform and continuous coating of certain polymer and metal substrates having relatively low limits of acceptable temperature exposure.